[RFC PATCH] mm/highmem: VM_BUG_ON() if offset + len > PAGE_SIZE

From: "Fabio M. De Francesco" <fmdefrancesco@gmail.com>
To: Ira Weiny <ira.weiny@intel.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	"Matthew Wilcox (Oracle)" <willy@infradead.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	Peter Collingbourne <pcc@google.com>,
	linux-kernel@vger.kernel.org
Cc: "Fabio M. De Francesco" <fmdefrancesco@gmail.com>
Subject: [RFC PATCH] mm/highmem: VM_BUG_ON() if offset + len > PAGE_SIZE
Date: Sun, 24 Apr 2022 12:48:06 +0200	[thread overview]
Message-ID: <20220424104806.25396-1-fmdefrancesco@gmail.com> (raw)

Call VM_BUG_ON() if offset + len > PAGE_SIZE.

This is an RFC patch because I'm not sure if we actually need to BUG_ON(),
so I'd like to know what Maintainers and other developers think about this
proposal.

Please note that memset_page() uses VM_BUG_ON() to prevent the same issue.

Furthermore, I also cannot understand why memset_page() does not call
flush_dcache_page() after memset() but I suppose that the different
implementations of these two functions should be addressed in another RFC.

Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
---
 include/linux/highmem.h | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/include/linux/highmem.h b/include/linux/highmem.h
index c3d562b5f0c1..bdb16d5f8148 100644
--- a/include/linux/highmem.h
+++ b/include/linux/highmem.h
@@ -349,6 +349,8 @@ static inline void memcpy_to_page(struct page *page, size_t offset,
 static inline void memzero_page(struct page *page, size_t offset, size_t len)
 {
 	char *addr = kmap_local_page(page);
+
+	VM_BUG_ON(offset + len > PAGE_SIZE);
 	memset(addr + offset, 0, len);
 	flush_dcache_page(page);
 	kunmap_local(addr);
-- 
2.34.1

